Lean-burn spark-ignition engines are primarily supplied with, and combust, a lean mixture of air and fuel (oxygen-rich mixture) to achieve more efficient fuel economy. The exhaust emitted from such engines during periods of lean-burn operation may include a relatively high content of nitrogen (N2) and oxygen (O2), a relatively low content of carbon monoxide (CO) and unburned/partially-burned hydrocarbons (HC's), and small amounts of nitrogen oxides primarily comprised of NO and NO2 (collectively referred to as NOX). The NOX constituency of the exhaust may fluctuate between about 50 and 1500 ppm and generally comprises far greater amounts of NO than NO2 along with nominal amounts of N2O. The hot engine exhaust, which can reach temperatures of up to about 900° C., often needs to be treated before it can be released to the atmosphere.
An exhaust aftertreatment system may be associated with the lean-burn engine to help remove unwanted gaseous emissions that may be present in the lean-burn engine exhaust. The exhaust aftertreatment system may be configured to receive an exhaust flow from the lean-burn engine and generally aspires to cooperatively (1) oxidize CO into carbon dioxide (CO2), (2) oxidize HC's into CO2 and water (H2O), and (3) convert NOX gases into N2 and O2. The reduction of NOX to N2 is generally the most difficult exhaust reaction to facilitate since the hot, oxygen-abundant, and low reductant content nature of lean-burn engine exhaust renders the kinetics for that reaction quite unfavorable. A variety of exhaust aftertreatment system architectures that employ specially-catalyzed components can nonetheless sufficiently facilitate the removal of CO, HC's, and NOX so that the exhaust expelled to the environment contains a much more desirable chemical makeup.
A NH3—SCR catalyst, for example, may be included in the exhaust aftertreatment system to help reduce NOX to N2. The NH3—SCR catalyst may be washcoated onto a support substrate and located in the flow path of the exhaust. Ammonia may be introduced into and mixed with the exhaust emanated from the lean-burn engine upstream from the NH3—SCR catalyst. One way to introduce NH3 into the lean-burn engine exhaust is to periodically combust a stoichiometric or rich mixture of air and fuel in the lean-burn engine and to pass the resulting rich-burn engine exhaust through a catalytic converter that comprises a three-way-catalyst to passively generate NH3 from native NOX and H2. The NH3 is then absorbed by the NH3—SCR catalyst where it becomes available to selectively reduce NOX contained in the engine exhaust to N2 in the presence of O2. Unreacted or excess NH3 may remain absorbed by the NH3—SCR catalyst for consumption at a later time. The passive generation of NH3 from native NOX can, in some instances, obviate the need to store NH3 or urea in an on-board storage tank that requires monitoring, regular refilling, and the active dosing of NH3 or urea into the exhaust through an injector device.
The molar ratio of NO to NO2 in the exhaust fed to the NH3—SCR catalyst may affect low-temperature NOX conversion. Many NH3—SCR catalysts convert NOX to N2 more efficiently when the molar ratio of NO to NO2 is significantly lower than that produced by the lean-burn engine. A lower NO to NO2 molar feed ratio may be achieved by positioning an oxidation catalyst that oxidizes NO to NO2 upstream of the NH3—SCR catalyst. Conventional oxidation catalysts such as a diesel oxidation catalyst or a two-way catalyst generally comprise a combination of platinum group metals (PGM's). But the PGM's used to prepare the oxidation catalyst material—most notably platinum and palladium—are quite expensive. Several of the PGM's used have also been shown, in some instances, to exhibit poor thermal durability and to lose some catalytic activity when exposed to high-temperature engine exhaust.
The use of a NH3—SCR catalyst in the exhaust aftertreatment system for a lean-burn engine is an attractive, yet challenging, option for removing NOX from the engine's exhaust. Such a device is often paired with an upstream oxidation catalyst to boost NOX conversion at low-temperatures. Conventional oxidation catalysts, however, primarily contain expensive and insufficiently durable PGM's. Exhaust aftertreatment and NH3—SCR technology related to NOX removal are thus constantly in need of innovative developments that can help advance to this and other related fields of technological art.